As a transporting the body for transporting developing agent to a developing portion, a developing roller with a circular cross-section is generally used, which roller is contacted by a rotating developing agent supplying roller so that the developing agent contained in a developing unit is supplied to the surface of the developing roller.
Therefore, in order to form a thin uniform layer of the developing agent on the surface of the developing roller, it is essential that the developing agent supplying roller supply the developing agent, an optimal amount at a time, to a contact portion contacted by the developing roller. Physical properties of the developing agent supplying roller thus becomes a very decisive factor.
As a material for such a developing agent supplying roller, polyurethane foam resin imparted with conductivity is used in the conventional technology. However, little attention is paid to the relationship between various physical properties of the material and the uniformity of the thickness of thin film of developing agent formed on the surface of the developing roller, except that a material of a suitable hardness is normally selected.
The supplying of the developing agent to the surface of the developing roller was performed, in the conventional technology, in such a way that the developing agent supplying roller was rotated at a peripheral speed (rotational speed measured at the edge of the roller) between 0.5 and 1 times that of the developing roller.
In the conventional technology, however, little consideration is given to the physical properties of the material of the developing agent supplying roller. In addition, the peripheral speed of the developing agent supplying roller is determined mainly in consideration of the occurrence of the splashing of developing agent inside a developing unit. Therefore, the purpose of making uniform the thickness of a thin film formed on the surface of the developing roller is not served.
Consequently, the conventional image forming device has a disadvantage in that, since the amount of developing agent carried by the developing agent supplying roller cannot be optimized, a thin film cannot be formed with uniform thickness on the surface of the developing roller, thereby causing bad quality printing, print history in which a printing pattern of the previous printing is retained, and uneveness in the darkness of printing.
Accordingly, the object of the present invention is to provide an image forming device having a developing agent supplying body capable of supplying an optimal amount of developing agent to a developing agent transporting body for transporting developing agent to a developing unit.
Another object of the present invention is to provide an image forming device capable of forming, on the surface of a developing agent transporting body, a uniform thickness thin film layer, of developing agent, by optimizing physical properties of the developing agent supplying body.
Still another object of the present invention is to provide an image forming device capable of forming, on the surface of a developing agent transporting body, a uniform thickness thin film layer of developing agent, by optimizing a moving speed of the developing agent supplying body, which speed is measured at a contact portion contacted by the developing agent transporting body.